Burners such as those employed in domestic water heaters are typically formed of two plates having a chamber between them when the plates are fastened together. The burner includes a central axis and the upper and lower plates include outer peripheries that are radially spaced from the central axis. The lower plate includes an opening for air and fuel to enter the chamber. A bracket is fastened to the lower plate for securing the burner in place. A plurality of ports extend radially outwardly from the chamber.
The products of complete combustion are carbon dioxide, water and, in the presence of excess air, nitric oxides, which include nitric oxide (NO) and nitrogen dioxide (NO.sub.2) (collectively referred to as NOx). The primary source of nitrogen is the air, both primary and secondary, required to combust gaseous fuels. The mechanism for formation of oxides of nitrogen is dissociation of nitrogen during the combustion process at flame temperatures in excess of 2600.degree. F. and reaction with oxygen present. Nitrogen dioxide in particular is undesirable in that it is a toxic gas and a major constituent of smog. Some states have laws that require water heater burners to have low levels of NOx not greater than 40 Ng/j. Therefore, burners are designed to reduce NOx emissions and achieve suitable combustion efficiency.
One way that gas burners have been designed in an attempt to reduce NOx emissions is to position a portion of the burner in a flame hot zone. This is intended to lower the temperature of the combustion reaction to a point below that which is conducive to NOx formation. Bumer portions placed in the path of the flame in an attempt to reduce flame temperature include rods and a descending peripheral edge of the upper plate, as disclosed in U.S. Pat. No. 5,913,675. However, such burners are typically effective in only specific water heaters, and do not necessarily produce the same emissions when applied to heaters made by various manufacturers. The location of the flame hot zone is believed to move as a result of different conditions including input rate, distance the buner is located from the tank wall, shape of the combustion chamber and the method and amount of secondary air introduced into the combustion chamber. When the location of the hot zone changes due to a change in flame geometry such that the burner portion is not positioned in the flame hot zone, NOx emissions rise above legislated levels. This renders such burners unacceptable for use in a variety of water heaters.
There is a need in the industry for a gas burner which achieves acceptably low NOx emissions when used in a variety of sizes and in water heaters made by various manufacturers, while achieving suitable combustion efficiency.